With high-speed wireless services increasingly in demand, there is a need for more throughput per bandwidth to accommodate more subscribers with higher data rates while retaining a guaranteed quality of service (QoS). In point-to-point communications, the achievable data rate between a transmitter and a receiver is constrained by the available bandwidth, propagation channel conditions, as well as the noise-plus-interference levels at the receiver. For wireless networks where a base-station communicates with multiple subscribers, the network capacity also depends on the way the spectral resource is partitioned and the channel conditions and noise-plus-interference levels of all subscribers. In current state-of-the-art, multiple-access protocols, e.g., time-division multiple access (TDMA), frequency-division multiple-access (FDMA), code-division multiple-access (CDMA), are used to distribute the available spectrum among subscribers according to subscribers' data rate requirements. Other critical limiting factors, such as the channel fading conditions, interference levels, and QoS requirements, are ignored in general.
Recently, there is an increasing interest in orthogonal frequency-division multiplexing (OFDM) based frequency division multiple access (OFDMA) wireless networks. One of the biggest advantages of an OFDM modem is the ability to allocate power and rate optimally among narrowband sub-carriers. OFDMA allows for multi-access capability to serve increasing number of subscribers. In OFDMA, one or a cluster OFDM sub-carriers defines a “traffic channel”, and different subscribers access to the base-station simultaneously by using different traffic channels.
Existing approaches for wireless traffic channel assignment are subscriber-initiated and single-subscriber (point-to-point) in nature. Since the total throughput of a multiple-access network depends on the channel fading profiles, noise-plus-interference levels, and in the case of spatially separately transceivers, the spatial channel characteristics, of all active subscribers, distributed or subscriber-based channel loading approaches as fundamentally sub-optimum. Furthermore, subscriber-initiated loading algorithms are problematic when multiple transceivers are employed as the base-station, since the signal-to-noise-plus-interference ratio (SINR) measured based on an omni-directional sounding signal does not reveal the actual quality of a particular traffic channel with spatial processing gain. In other words, a “bad” traffic channel measured at the subscriber based on the omni-directional sounding signal may very well be a “good” channel with proper spatial beamforming from the base-station. For these two reasons, innovative information exchange mechanisms and channel assignment and loading protocols that account for the (spatial) channel conditions of all accessing subscribers, as well as their QoS requirements, are highly desirable. Such “spatial-channel-and-QoS-aware” allocation schemes can considerably increase the spectral efficiency and hence data throughput in a given bandwidth. Thus, distributed approaches, i.e., subscriber-initiated assignment are thus fundamentally sub-optimum.
Linear Modulation Techniques, such as Quadrature phase shift keying (QPSK), Quadrature Amplitude Modulation (QAM) and multi-carrier configurations provide good spectral efficiency, however the modulated RF signal resulting from these methods have a fluctuating envelope. This puts stringent and conflicting requirements on the power amplifier (PA) used for transmitting communications. A fluctuating envelope of the modulating signal requires highly linear power amplification. But in order to achieve higher efficiency and improve uplink budget, power amplifiers have to operate close to compression and deliver maximum possible power. As a result, there is a trade off for power versus amount of nonlinear amplification a system can handle.
Furthermore, non-linearity in the PA generates intermodulation distortion (IMD) products. Most of the IMD products appear as interference to adjacent channels. This power is referred to Adjacent Channel Leakage Power Ratio (ACPR or ACLR) in wireless standards.
The ACPR is important to the FCC and wireless standards because of the co-existence with other users of the spectrum operating in adjacent and alternate channels. In band or channel distortion affects the performance of the licensee's own spectrum, which, in turn, affects the transmitter signal-to-noise ratio (SNR) of other users in the same system.
RF link budget in a wireless communication system refers to balancing the available transmit power, antenna gain, propagation loss and determining maximum allowable distance at which received power meets a minimum detectable signal threshold. Several parameters influence the RF link budget. Two main factors, transmitter RF power available from the PA and receiver sensitivity, are under circuit designer's control. Base station design has relatively more degree of freedom than the Customer Equipment (CE). This results in the RF link budget being imbalanced in the uplink: This limitation is hard to overcome given the cost, size and battery life requirements of CE.